Leads implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life threatening arrhythmias, or to stimulate contraction (pacing) of the heart. Electrical energy is applied to the heart via the leads to return the heart to normal rhythm. Leads have also been used to sense in the atrium or ventricle of the heart and to deliver pacing pulses to the atrium or ventricle. The same lead used to sense the condition is sometimes also used in the process of delivering a corrective pulse or signal from the pulse generator of the pacemaker.
Cardiac pacing may be performed by the transvenous method or by leads implanted directly onto the ventricular epicardium. Most commonly, permanent transvenous pacing is performed using a lead positioned within one or more chambers of the heart. A lead, sometimes referred to as a catheter, may be positioned in the right ventricle or in the right atrium through a subclavian vein, and the lead terminal pins are attached to a pacemaker, which is implanted subcutaneously. The lead may also be positioned in both chambers, depending on the lead, as when a lead passes through the atrium to the ventricle. Pacing and sensing electrodes may be positioned within the atrium or the ventricle of the heart.
Pacemaker leads represent the electrical link between the pulse generator and the heart tissue, which is to be excited. These pacemaker leads include single or multiconductor coils of insulated wire having an insulating sheath. The coils provide a cylindrical envelope, many times referred to as a lumen, which provides a space into which a stiffening stylet or guidewire can be inserted. The conductive coil is connected to an electrode in an electrode assembly at a distal end of the lead.
During use, the lead conducts critical information to and from the heart. The lead, therefore, must remain in sufficient operative condition without interference from entry of bodily fluids. A lumen used as a means for deployment must remain free of blood to remain functional. Clotting blood hampers maneuverability. In addition, blood in the lumen could cause corrosion. To prevent entry of bodily fluids into the lead, a seal can be provided at the distal end of the lead. Conventional leads do not have open ends, i.e. the lumens thereof are closed. Leads which have moving parts, such as retractable fixation parts for example, use O-ring type seals to seal the distal end of the lead from entry of bodily fluids. The O-ring seals can be difficult to manufacture due to dimensional constraints which affect the effectiveness of the seal and the ease with which parts move.
Accordingly, there is a need for a lead, which is sufficiently sealed from the environment. There is also a need for a reliable means for performing cardiac sensing and pacing of the left atrium or ventricle, with a lead which is sufficiently sealed from the environment.